There are two different types of ceramic tiles-glazed and unglazed. Sealing of glazed ceramic tiles is not necessary as they are non-porous. The only thing that needs to be sealed in that case is the grout line which you would do after the tiles are laid.
In the instance of non-glazed ceramic tiles, or any other type of tile or stone, you can use MR Seal prior to grouting as a grout release and after grouting to complete the sealing process. A light spray on the tiles prior to grouting facilitates easier cleanup of grout and prevents grout stains on the tiles. After tiles are laid and grouting is finished, an additional application of MR Seal should be done per can instructions on the tiles and grout lines.